1. Field of the Invention
This invention relates to removing coatings from cathode ray tubes, for example cathode ray tubes intended to act as picture tubes of television receivers, or parts of such tubes.
2. Description of the Prior Art
Cathode ray tubes (CRTs) intended to act as picture tubes of color television receivers conventionally are constructed by various different manufacturers in a manner that will now be described with reference to FIG. 1 of the accompanying drawings. The CRT is formed from a funnel-like glass body 10, referred to hereinafter more simply as a "funnel", and a glass face plate or panel 12. A metal electrode member 14, known as an anode button, extends through the wall of a main body portion 18 of the funnel 10 and is sealed in a vacuum-tight manner to the glass making up the wall.
To make the CRT, a carbon (graphite) coating (not shown) is applied to the interior of the funnel 10. The composition of the internal graphite coating may vary as between the main body portion 18 and a neck portion 20 which is designed to accommodate an electron gun (not shown). The funnel 10 then is heated in an oven so that the internal carbon coating will cure or set.
The funnel 10 then is joined to the panel 12, the interior of the panel previously having been coated in respective local areas with one or more phosphors, its own carbon (graphite) coating and, in some instances, aluminum. The funnel 10 is joined to the panel 12 by applying an adhesive to an end edge 22 of the funnel and/or to an edge 24 of the panel 12 so that the edges 22 and 24 are joined together by the adhesive. The adhesive comprises a frit and a binder liquid and, after it has been applied, the assembly of the funnel 10 and panel 12 is heated so that the adhesive will cure or set to firmly bond together the funnel 10 and panel 12 in such a manner as to provide a vacuum-tight seal between them.
Thereafter, the funnel 10 is evacuated and, whilst it is evacuated, the electron gun (not shown) is sealingly fitted into the neck portion 20 of the funnel and the flared end part of the neck portion is removed. Also, a carbon (graphite) coating (not shown) is applied to at least part of the exterior of the main body portion 18 of the funnel 10. The exterior graphite coating may be of a different composition to that or those on the interior of the funnel 10.
The carbon coatings as applied to the inside and outside of the funnel 10 all comprise carbon (graphite) in a liquid binder and can be applied to the respective portions of the funnel by spraying or rolling, after which they are cured or set by heating. As is known to these skilled in the art of CRT manufacture, the various different coatings are available in liquid form from specialist suppliers, for instance from Acheson Colloids under the trademark "ELECTRODAG". The precise composition of the various carbon coatings will be determined by various factors, including the size of the CRT and its intended quality. By way of example, however, the coating as applied to the interior of the main body portion 18 of the funnel 10 may be (in parts by weight): 22% graphite, 10% potassium silicate and 68% water; while that applied to the interior of the neck portion 20 may be: 1 kg of graphite and 220 g of silicon carbide to 750 ml of water glass.
It sometimes happens that, for one reason or another, a CRT manufactured by the method just described will be found to be defective. For example, the seal formed by the adhesive joining together the funnel 10 and panel 12 may be defective so that a vacuum cannot be maintained in the CRT. Instead of throwing away the tube, many manufacturers will, in this event, try to salvage the funnel 10 and/or the panel 12 by separating them, whereupon the salvaged component can be reintroduced into the manufacturing process at a suitable stage and used in making another CRT.
The salvaged funnel 10 and/or panel 12 must be stripped of its coatings before it can be used. That is to say, the carbon (graphite) coatings must be removed from the interior and exterior of the funnel 10 and/or the carbon, phosphors and aluminum must be removed from the panel 12. According to a technique used by several manufacturers, the carbon coatings of the funnel 10, which are soluble in hydrofluoric acid (HF), are removed by spraying the interior and exterior of the funnel with HF. This technique has the disadvantages that it is time consuming (the HF taking about 20 minutes or so to remove the coatings), and that HF is both expensive and, by virtue of its corrosive nature, difficult and dangerous to handle.
In some cases, a manufacturing defect may be rectified without disassembling the funnel 10 and panel 12 from one another. For example, the carbon coating on the exterior of the funnel 10 of a completed CRT may be found to be defective, in which case that coating only needs to be removed and the CRT then can be put back on to the production line at an appropriate stage for a fresh external carbon coating to be applied. However, before this is done, the old (defective) coating must be removed. Alternatively, the electron gun of a completed tube may be found defective, in which case the tube is let down to air (that is, the vacuum is released), the gun is removed, a tubular piece of glass is welded to the end part of the neck portion 20 of the funnel 10, a new gun is fitted and the CRT can then be put back on the production line at an appropriate stage. Again, before the CRT is put back on the production line, the carbon coating on the exterior of the funnel must be removed. In either of the cases mentioned above, the external carbon coating can be removed by hand by scrubbing it off with wire wool and warm water. This is a time consuming process, taking typically about 7 to 8 minutes. Since the operation is performed by hand, it cannot be performed within a protective enclosure, whereby there is a risk of injury to the operator should the CRT accidentally be smashed during the course of the process. This danger is more acute if the operation is performed with the tube under vacuum, since smashing of the tube will in this event cause an implosion. (It will of course be appreciated that the tube is always under vacuum in the event that all that is required is the replacement of a defective external carbon coating. Further, even if a gun is to be replaced, it is in general preferable for the carbon coating to be removed before the coating is let down to air in order to reduce the possibility of any containment, for example the water used in the scrubbing process, from entering the CRT).